[fix] #54 fix gitlab ci to install python 3.6.8 with pyenv

fixme: python version should be pre installed in docker image

.gitlab-ci.yml

@@ -35,6 +35,8 @@ push_to_github:
     - export PYENV_ROOT="$HOME/.pyenv"
     - export PATH="$PYENV_ROOT/bin:$PATH"
     - eval "$(pyenv init -)"
+    # fixme: pre-install python version in docker image to gain speed
+    - pyenv install $PYENV_PYTHON_VERSION
     - pyenv shell $PYENV_PYTHON_VERSION
 
 .changes: &changes